The invention relates to a magneto-optical recording medium comprising a substrate on which a magneto-optical recording layer is provided in the form of a multilayer having an easy axis of magnetization extending perpendicularly to the plane of the layer, said multilayer being composed of layers of a first type which contain predominantly Co and layers of a second type which contain a metal other than Co, said layers being alternately provided.
In a MO (=magneto-optical) recording medium data is represented by magnetized domains or bits. The direction of magnetization of the domains extends perpendicularly to the plane of the MO recording layer. The bits can be detected by means of the MO effect in which the direction of polarization of polarized light changes as a function of the direction of magnetization of the domains when said light is reflected (Kerr-effect) or passed (Faraday-effect).
The magnetized domains are obtained by locally heating the MO recording layer to a temperature above the Curie temperature of the material of the MO recording layer by means of a focused laser light beam. During cooling to a temperature below the Curie temperature, the magnetization is locally oriented according to the direction of an externally applied magnetic field and then frozen. This method is also termed thermomagnetic recording. A customary manner of inscribing information in a MO recording layer consists in rotating a disc-shaped recording medium while a permanent external magnetic field and a laser light beam which is modulated according to the information to be recorded inscribe magnetic domains in the MO recording layer. In this method the existing information has to be erased first before new information can be inscribed. A method of overwriting existing information with new information (termed direct overwrite) is magnetic field modulation (MFM). In MFM the external magnetic field is modulated according to the information to be recorded, the focused laser light beam used having a constant power. The external magnetic field is generated by a magnet coil.
A material which is customarily used for the MO recording layer is formed by alloys of rare earth transition metals such as GdTbFe and TbFeCo. Said materials have a number of disadvantages such as their sensitivity to oxidation and corrosion and their relative insensitivity to relatively short wavelengths of the laser light used. An alternative material for the MO recording layer which does not have the above disadvantages is a multilayer formed by alternating very thin layers of Co and Pt (or Pd).
Such an MO recording medium is described in European Patent Application EP-A-304873. The MO recording layer is composed of alternating layers of Co and Pt (or Pd). The Co layers have a thickness which ranges between 0.1 and 0.9 nm and the Pt layers have a thickness which ranges between 0.3 and 4 nm. The number of layers is selected such that the overall layer thickness of the MO recording layer is 5-80 nm.
Important properties for MO recording are, inter alia, the value of the magnetization and the Curie temperature of the MO recording layer. Said properties can be changed to a limited degree by varying the Pt layer thickness. Both the magnetization and the Curie temperature of the MO recording layer decrease according as the Pt layer thickness increases. The most favourable Carrier-to-Noise Ratio (CNR) is obtained, however, by using multilayers having relatively thin Pt layers, for example, of a thickness varying between 1.0 and 1.5 nm; a Co layer thickness of approximately 0.4 nm and an overall layer thickness of the recording layer of approximately 20 nm which corresponds to 14 Co/Pt layer combinations. However, such a multilayer simultaneously exhibits a relatively high magnetization and a high Curie temperature. Owing to said high Curie temperature the multilayer must be heated to a relatively high temperature during the recording process, which limits the possibility of rewriting. As a result of said high magnetization, high magnetic fields must be used during writing, which is disadvantageous, in particular, for writing using MFM having a high frequency (order of magnitude of 10 MHz). In the above-mentioned European Patent Application various elements are stated which can be added to the Co layer to reduce the Curie temperature of the MO recording layer. Examples of said elements are W, Nb, Ta and Mo. The addition of said elements to the Co layer has a number of disadvantages. Thus, already at low concentrations of said additives the nucleation field H.sub.n (in kA/m) becomes negative as a result of which the remanence is not 100% and, hence, the hysteresis loop is no longer square. The nucleation field H.sub.n is the magnetic field at which nucleation of a domain of opposite magnetization starts. A MO recording layer having a magnetic remanence below 100% exhibits a lower output (Kerr-effect) signal and, besides, additional noise (media noise). In addition, the magnetization may be disturbed by fields which are weaker than the coercive field H.sub.c (in kA/m), which fields may occur during inscribing a nearby domain.
It is an object of the invention to provide, inter alia, a MO recording medium in which the Curie temperature and the magnetization of the MO recording layer are reduced but in which the nucleation field H.sub.n remains positive and, hence, the hysteresis loop remains square. A reduction of the Curie temperature leads to an improved rewritability and a reduction of the magnetization enables the use of higher write frequencies.
According to the invention this object is achieved by a MO recording medium as described in the opening paragraph, which is charactexized in that the Co-containing layers comprise an element which is selected from the group formed by Os (osmium) and Re (rhenium). The addition of one of these elements to the Co layers leads to both a reduction of the Curie temperature and to a lower magnetization of the MO recording layer. In the case of an effective addition of said elements and at a customary overall multilayer thickness the influence on the nucleation field H.sub.n is small so that also the magnetic remanence remains 100%. The addition of elements to the Co layer does lead, however, to a lower Kerr rotation and, hence, also to a lower carrier level during reading so that an optimum must be found between the drawback of a lower carrier level and the advantages of a lower Curie temperature and a lower magnetization.
As noted above, the addition of elements such as W, Nb, Ta or Rh to the Co layer leads, depending on the concentration and the overall multilayer thickness, to a negative nucleation field H.sub.n, while the reduction of the Curie temperature is small. Elements such as Ir, Ru and Mo yield a moderate reduction of the nucleation field but the effect on the Curie temperature is very small.
Only by using Os and Re a sufficient reduction of the Curie temperature and the magnetization are attained in combination with a very small reduction of the nucleation field H.sub.n.
The optimum addition of Os or Re to the Co layers is governed by the overall multilayer thickness and is maximally 50 at. %. Higher concentrations lead to a negative nucleation field H.sub.n. A suitable concentration is 3-8 at. % at an overall multilayer thickness of 20 nm.
Co/Pt multilayers can be prepared at room temperature by means of sputtering or by means of electron beam evaporation from separate Co and Pt evaporation sources. The addition of Os or Re takes place from a third sputter target or evaporation source. Alternatively, a sputter target can be used which consists of an alloy of Co and Os (or Re) thus producing layers consisting essentially of Co and Os (or Re), and of Pt.
The application of the above-mentioned invention is not limited to Co/Pt and Co/Pd multilayers as described in the above-mentioned EP-A-304873 but it can also be applied when Co/Au multilayers as described in EP-A-376375 are used and when Co/Ni multilayers as described in Netherlands Patent Application NL 9002840 are used, the last two Patent Applications being filed by Applicants, and the invention can also be applied when Co/Ru multilayers are used.
A metallic non-magnetic underlayer of, for example, Au, Pd or Pt may be applied between the substrate and the multilayer. The thickness of such an underlayer is, for example, 1-50 nm. By virtue of such an underlayer, in particular, H.sub.c is increased and, in addition, the perpendicular anisotropy of the multilayer is improved.
Exposure to the focused laser light beam is preferably carried out via the substrate. In this case, the substrate, which is mostly disc-shaped, must be transparent to the wavelength of the laser light used. In this manner, it is precluded that any dust particles or other impurities present on the surface of the substrate adversely affect the quality of recording and reading because said impurities are remote from the focal point of the objective used to focus the light beam on the multilayer. The transparent substrate is manufactured from, for example, glass, quartz or a transparent synthetic resin such as polycarbonate or polymethyl methacrylate. The surface of the substrate at the side of the multilayer may be provided with an optically scannable groove, a so-called servotrack or guide track, which is generally spiral-shaped. It is alternatively possible to use a replication process to provide the groove in a separate synthetic resin layer applied to the substrate, which layer consists of, for example, a UV light-cured layer of acrylates. The MO layer is applied on top of said layer. The outermost MO layer can be shielded from its surroundings by a protective layer of, for example, UV light-cured acrylate.
Inorganic dielectric layers, socalled tuning or enhancement layers, may be provided between the underlayer and the substrate and between the multilayer and the protective layer. Such layers increase the Kerr rotation. Examples of such dielectric layers are oxides such as SiO.sub.2 and TiO.sub.2, and nitrides such as AlN, AlSiN and Si.sub.3 N.sub.4.
The linear velocity of the MO recording medium is governed by the type of information which is recorded, such as audio, video or data information and is, for example, 1 to 20 m/s. During recording, the exposed areas of the multilayer are heated to a temperature above or close to the Curie temperature, after which a magnetic domain or bit is formed in the multilayer by cooling, said domain or bit having a direction of magnetization which corresponds to the external magnetic field. Said external magnetic field originates from a magnet coil and is modulated in accordance with the information to be recorded with a frequency in the MHz range. The bits formed have small diametral dimensions of maximally one or a few micrometers so that a high information density is obtained.
The recorded information is read by using linearly polarized laser light having a low power and reading is based on the measurement of the Kerr-effect.
The invention will be explained in greater detail by means of exemplary embodiments and with reference to the accompanying drawings.